Messages in this thread | | | Subject | Lockless file reading | From | Timo Sirainen <> | Date | Wed, 27 Aug 2003 15:37:18 +0300 |
| |
(Maybe a bit off topic, but I couldn't get answers elsewhere and it is about kernel behaviour)
The question is what can happen if I read() a file that's being simultaneously updated by a write() in another process? If I'm writing 123 over XXX, is it possible that read() returns 1X3 in some conditions? Is the behaviour filesystem specific? Any idea about other operating systems?
I'm thinking about implementing lockless file reads that work like:
void write_data(int fd, off_t offset, void *data, size_t size) { lock_file(fd); pwrite(fd, data, size, offset); // or writev() or copy+single pwrite() pwrite(fd, data, size, offset + size); unlock_file(fd); }
void read_data(int fd, off_t offset, void *data, size_t size) { unsigned char buf{size*2]; for (;;) { pread(fd, buf, size*2, offset); // or shared mmap()ed access if (memcmp(buf, buf+size, size) == 0) break; usleep(10); } memcpy(data, buf, size); }
The only case when I see that it would break is if we write "1212" but pread() sees "1X1X" or "X2X2" there.
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |